The relative significance of different pathways of phenylalanine metabolism in the animal body will be examined in vivo by studying carbon dioxide production, urinary aromatic metabolite excretion and activities of phenylalanine hydroxylase, phenylalanine-pyruvate amino transferase and aromatic amino acid decarboxylase in rats that have been administered loads of variously C14-labeled phenylalanine. The metabolism of branched-chain amino acids and antagonistic effects among the branched-chain amino acids will be examined in isolated perfused muscle preparations. Branched-chain keto acid production will be monitored after a variety of nutritional treatments known to alter the utilization of branched-chain amino acids by animals in vivo. BIBLIOGRAPHIC REFERENCES: Maruyama, K., A.E. Harper and M.L. Sunde. Effects of D-, DL- and L-Glutamic Acid on Chicks. J. Nutr. 105: 1012-1019 (1975). Ip, C. and A.E. Harper. Protein synthesis in Liver, Muscle, and Brain of Rats Fed a High Tyrosine-Low Protein Diet. J. Nutr. 105: 885-893 (1975).